It is well known that maximum activity of the Group VIII metals for hydrogenation reactions depends upon maintaining the metal in a finely divided state such that there is a maximum ratio of surface area to mass. Perhaps the most common method of achieving a high degree of dispersion involves impregnating salts of the Group VIII metals upon porous solid supports, followed by drying and decomposing of the impregnated salt. On non-zeolitic supports, the drying and calcining operations often bring about a substantial migration and agglomeration of the impregnated metal, with resultant reduction in activity. In more recent years, with the advent of highly active crystalline zeolite catalysts of the aluminosilicate type, it has become common practice to ion exchange the desired metal salt into the zeolite structure in an attempt to achieve an ionic bond between each metal atom and an exchange site on the zeolite, thus achieving the ultimate in dispersion of metal while also bonding the metal to the zeolite in such manner as to minimize migration and agglomeration during the drying and calcining steps. This ion exchange technique is particularly desirable in the case of dual-function catalysts such as hydrocracking catalysts wherein it is desirable to maintain an active hydrogenating site closely adjacent to an acid cracking site. These efforts have met with varying degrees of success.
Even though the above described ion-exchange techniques can give a high degree of initial dispersion of metal on the support, conditions encountered during subsequent use of the catalyst may bring about a maldistribution of the metal with resultant reduction in activity, independently of normal deactivating phenomena such as coking, fouling, poisoning, etc. Overheating, or contact with excessive partial pressures of water vapor at high temperatures, such as may occur during oxidative regeneration of the catalyst, may bring about migration of the active metal away from the exchange sites, and this migration may ultimately result in substantial agglomeration of the Group VIII metal. Such agglomeration is not reversible by conventional oxidative regeneration techniques.
In U.S. Pat. No. 3,899,441 to Hansford a method is disclosed for at least partially redispersing Group VIII metals which have become agglomerated as above described. According to this method, the deactivated catalyst, in an oxidized or sulfided state, is subjected to a hydration-ammoniation treatment involving the adsorptive saturation of the catalyst with water vapor and ammonia, followed by careful drying and calcination to effect deammoniation. This treatment brings about a substantial redispersal of the Group VIII metal, with resultant recovery of most or all of the hydrogenation activity not recoverable by conventional oxidative regeneration.
The mechanism by which the Hansford rejuvenation process operates is believed to involve formation of a soluble metal ammino-hydroxide in the pores of the catalyst. Hydration and ammoniation of the deactivated catalyst fills the micropores with a strong aqueous ammonia solution, resulting in dissolution of the metal oxide or sulfide in the ammonia solution in the form of a soluble ammino-hydroxide. For example, palladium oxide on the zeolite support will form the Pd(NH.sub.3)4++ ion, which then migrates back to the original ion exchange sites. The original distribution of palladium is then theoretically obtained after drying and recalcining. Similarly, platinum oxide on amorphous silica-alumina will form Pt(NH.sub.3).sub.4 (OH).sub.2 or Pt(NH.sub.3).sub.6 (OH).sub.4 which, being stronger bases than NH.sub.4 OH, will tend to combine with the original acid sites on the support. The original distribution of platinum with respect to acid sites will then theoretically be obtained after drying and calcining.
In practicing the rejuvenation process of Hansford, the degree of recovery of fresh activity of the catalyst depends to a large extent upon the extent of agglomeration which has taken place. In cases where metal agglomeration is minimal, the Hansford process can effect a complete recovery of fresh activity, and even in some cases better than fresh activity. In the case of severely damaged catalysts however it has been found very difficult to achieve complete recovery of fresh activity with the Hansford process alone. I have now discovered however that, following the hydration-ammoniation-calcination rejuvenation of Hansford, a substantial additional recovery of activity can be achieved by the simple expedient of partially rehydrating the catalyst prior to final activation in hydrogen. This additional recovery of activity appears to involve some change in disposition of the Group VIII metal, but at present it is uncertain as to whether this change involves additional redispersal, or perhaps some type of migration of the metal to more desirable sites. A very puzzling aspect of the invention resides in the fact that complete rehydration followed by drying and calcination actually decreases the activity of the catalyst. It is hence a critical feature of the invention to rehydrate the catalyst only to an extent of less than about 80% of its adsorptive capacity for water, measured in terms of weight loss which the catalyst undergoes upon heating for 2 hours at 1000.degree. C, after first being equilibrated with water-saturated air at 70.degree. F.